A memory on any computing system is a limited resource. No matter how fast computing systems become, they always depend upon a finite amount of memory in which to run their software applications. As a result, software developers should consider this resource when writing and developing software applications.
The Java programming language differs from many traditional programming languages (e.g., C, C++) by the way in which memory is allocated and deallocated. In languages like C and C++, memory is explicitly allocated and deallocated by the application programmer/developer. This can greatly increase the time spent by programmers in tracking down coding defects in regards to deallocating memory. The Java programming language presents several features that appeal to developers, of large-scale distributed systems, such as “write once, run anywhere” portability, portable support for multithreaded programming, support for distributed programming, including remote method invocation, garbage collection, and an appealing object model have encouraged Java use for systems with a size and complexity far beyond small applets. However, the developers of these applications often encounter problems, such as memory leaks, performance and scalability problems, synchronization problems, and programming errors.
Java runtime environments (e.g., Java virtual machine) provide a built-in mechanism for allocating and deallocating memory. In Java, memory is allocated to objects. The Java virtual machine (“VM” or “JVM”) automatically handles the amount and allocation of memory upon an object's creation. The Java runtime environment employs a “garbage collector” (GC) to reclaim the memory allocated to an object that is no longer needed. Once the GC determines that the object is no longer accessible (e.g., when there is no longer any references to it stored in any variables, the fields of objects, or the elements of any arrays, etc.), it reclaims the allocated memory. When objects in a Java application are no longer referenced, the heap space the object occupied is to be recycled so that the space becomes available for subsequently-created objects.
Although having garbage collection improves productivity, it is not entirely immune from a class of bugs, called “memory leaks.” A memory leak can occur when a program (or in the case of Java, the VM) allocates memory to an object but never (or only partially) deallocates the memory when the object is no longer needed. As a result, a continually increasing block of memory may be allocated to the object, eventually resulting in an “Out Of Memory Error” (OOME). In other words, a memory leak occurs when memory is allocated, but it is never (or only partially) reclaimed. Memory leaks can also occur when a data structure (e.g., hashtable) is used to associated one object with another and even when neither object is required any longer, the association with the data structure remains, preventing the objects from being reclaims until the data structure is reclaimed. Stated differently, when a lifetime of the data structure is longer than that of the objects associated with it, memory leaks are caused.
Memory leaks are of particular concern on Java-based systems (e.g., Java 2 Platform Enterprise Edition (J2EE) platforms) which are to run twenty-four hours a day, seven days a week. In this case, memory leaks, even seemingly insignificant ones, can become a major problem. Even the smallest memory leak in code that runs 24/7 may eventually cause an OOME, which can bring down the VM and its applications or even all VMs running on a particular application server instance. This can cause critical performance problems.
It is generally preferred to profile memory use and debug memory leaks in an application code in the early stages of development to provide an early detection of memory problems long before the production stage. Although garbage collection makes code much safer, because having the developer to explicitly delete objects from memory is prone to human error, garbage collection is not a panacea. For example, if the developer does not manage the references to the Java objects carefully, it can result in a memory leak problem, such as a reference to an object is stored within an instance or class field, this reference may exist throughout the life of the application and, unless desired, is regarded a memory leak.
Within a distributed application server environment having thousand of concurrent users, performance and scalability problems are typical. The causes of problems are various, such as synchronization problems, extensive access to shared resources (e.g., database systems), bad configuration settings, etc. To provide consistency within such a system, locks with various validity scopes (e.g., VM-local, application-server-wide, and system-wide) are used; however, deadlock situations and synchronization problems exist.
Several performance monitoring, profiling, and debugging tools are used to examine software applications to determine resource consumption within the Java runtime environment (JRE). For example, a profiling tool may identify the most frequently executed methods and objects created in an application. A type of software performance and debugging tool is a “tracer.” However, such tools are very limited in detecting and exposing system inefficiencies and problems (e.g., memory leaks), while consuming great amounts of system resources by requiring overhead tasks, such as starting and restarting of VMs in special modes. Further, such tools are also limited in providing necessary information about system problems and the limited information that these tools may provide is not useful for applications comprising several thousand objects. This leaves developers with often insurmountable amounts of code to manually evaluate to track down the problem objects/variables, such as the specific class, method calls, etc. For example, conventional profiling tools, like Optimizelt and JProbe, when used, require restarting of VMs and servers, which results in loss of production and system resources, particularly when restarting a productive system. Moreover, the starting of a server and its VMs further adds to the system overhead by increasing memory consumption, which also harms the normal work of the server and server software. The restarting of the server adds overhead in regards to the Central Processing Unit (CPU), as the server would have to start up from scratch.
FIG. 1A illustrates a conventional profiling tool. Client 102 is in communication with server 108. Client 102 includes a VM 102. Server 108 includes a VM 112, which includes Java Virtual Machine Profiling Interface (JVMPI)-based interface 116 and implementation 114. Server 108 further includes a native/default profiling agent (having an agent library) 110 which is plugged into the VM 112 at start-up. Since JVMPI is a native/default-interface, the agent 110 is also written in native code. An agent 110 refers to a software entity, which is used to gather profiling information native VM interfaces (e.g., JVMPI). JVMPI-based implementation 114 suffers from high, memory footprints and, like conventional tools JProbe and Wily Introscope, requires a VM restart. However, conventional profiling tools (e.g., also those using Java Virtual Machine Tool Interface (JVMTI)) cannot be used in productive systems without disturbing user sessions. Further, they cannot be used in large application server environments as they cause high memory consumption. Referring back to FIG. 1A, for example, to start profiling traces, the VM 112 is to be restarted in special way, such as by having the agent 110 loaded at VM-startup, which can cause negative impact on performance and memory consumption. There are merely some of the limitations of conventional profiling solutions. Similarly, conventional monitoring tools and debugging tools (e.g., using Java Virtual Machine Debugging Interface (JVMDI)) also suffer from these and additional limitations.
FIG. 1B illustrates a hash table 150 having stack traces. While performing profiling of Java applications at a virtual machine, stack traces or stack trace elements (elements) 152-168 associated with the current thread are detected and saved in a hash table 150 in a VM. Since many of these stack trace elements 152-168 are repeatedly encountered, several of these elements 152-158 are repeatedly inserted into the hash table 150 which consumes a great deal of memory. Since the hash table 150 is a table-like structure that is used to record every single instance of a stack trace 152-158 that is encountered (and since most stack traces are repeatedly encountered), it consumes a great deal of memory. For example, the main stack trace element 152 (“H”), in the illustrated example, is encountered seven times and is recorded seven times. Similarly, stack trace element 154 (“A”) is encountered and is recorded each of the four times it is encountered. This problem extends even to external profiling, which also has to hold these stack traces in a hash table. Furthermore, each time a VM or a profiling tool at the VM needs to look for a profiling even with a stack trace 152-158, it needs to look at each of the stack traces 152-158 where that particular profiling event might be recorded because even if the event is common within various stack traces 152-158, it is recorded in the hash table 150 each time it is encountered, which consumes an even greater amount of memory and significantly contributes to inefficient profiling. Since most stack trace columns 170-182 are different only at their top as indicated by stack trace elements 154-158, 160-168 near the top of stack (TOS) 170, there is no need to repeat common stack traces, such as stack trace H 152; nevertheless, as illustrated, the hash tree 150 stores such stack traces repeatedly, consuming valuable memory space and contributing to profiling inefficiency.